Novel material

ABSTRACT

The present invention relates to a substrate comprising an ion-implanted layer, for example a cation, wherein the ion implanted layer has a uniform distribution of the implanted ions at a significantly greater depth than previously possible. The invention further comprises said substrate wherein the substrate is a silicon based substrate, such as glass. The invention also comprises the use of said material as a waveguide and the use of said material in measurement devices.

The present invention relates to a substrate comprising an ion-implanted layer, for example a cation, wherein the ion implanted layer has a uniform distribution of the implanted ions at a significantly greater depth than previously possible. The invention further comprises said substrate wherein the substrate is a silicon based substrate, such as glass. The invention also comprises the use of said material as a waveguide and the use of said material in measurement devices.

Femtosecond pulsed laser plasma deposition (fs-PLD) is a relatively new technique compared to its nanosecond pulsed counterpart. Technologically realized in a solid-state device in mid 90s, the fs-PLD is currently emerging as one of the promising technology in the field of thin film deposition due to the employment of femtosecond laser. Depending on the repetition rate and pulse duration in a fs laser-matter interaction can be tuned to become either a hot with heat diffusion or, a cold process due to the lack of heat diffusion process and hence, the laser is used for the production of plasma plume and energetic ions in several other techniques such as ion implantation. Recent reports in the fs-PLD thin films are mainly based on crystalline and semiconductor materials. At the present time implantation at a scale of only a few tens of nanometer, otherwise known as sub-plantation, have been reported. We have found that implantation to a much larger depth is possible. This observation has the potential to produce new materials and structures which are otherwise impossible to fabricate. The unique possibility of implanting ions, such as rare earth ions opens new realm of photonic devices engineering with respect to site selective doping by masking, direct printing of photonic circuits, integrated optical amplifiers in novel materials, multiple sensors with integrated pump source and data readouts, possibility of single chip multi-sensor design, superlattice structures by multi-target deposition.

International patent application, publication number: WO 2011/030157 describes a method of applying a film to a substrate by ablating a target with radiation from a laser whereby a quantity of the target layer is deposited on the substrate. Skelland and Townsend (1995) Journal of Non-Crystalline Solids 188, 243-253 describes ion-implantation into heated soda-lime glass substrates whereby the profile of the ions implanted shows a distinct peak with a gradual drop in ion density either side.

We have found a process which enables an ion layer to be implanted into a substrate, such as glass to a much greater depth than previously possible.

Thus, according to a first aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein the penetration depth of the implanted ions is at least 200 nm, for example at least 500 nm.

We have also found that the process provides a uniform distribution density of the implanted ions in the implanted layer rather than the density profile showing a peak followed by a drop off in implanted ion density, in a manner which the ion depths transcend conventional diffusion and high-energy ion implantation, due to major structural barriers for ion diffusion/implantation.

Thus, according to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein the ion implanted layer has a uniform distribution of the implanted ions.

We have also found that the process facilitates ion-implantation to a higher density than prior art processes.

Thus, according to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein the ion implanted layer has an implanted ion density of the order of 10,000 to several hundred thousands of ppm, depending on the concentrations of ions in the target material used for fs-laser ablation. For example, when a target of tellurite glass with 80 mol % TeO₂-10 mol % ZnO-9 mol % Na₂O and 1 mol % Er₂O₃ oxide was used to make laser-plasma implantation on a clean silica surface at 700° C., the implanted ion concentrations of Te⁴⁺, Na⁺, Zn²⁺, and Er³⁺ correspond to nearly half of the ion concentrations in the bulk target materials (e.g. 50 ion % Si⁴⁺, 40 ion % Te⁴⁺, 5 ion % of Zn²⁺, 4.5 ion % of Na⁺, and 0.5 ion % of Er³⁺). These densities are larger than 10²¹ ions cm⁻³. In conventional processes, the achievable densities are an order magnitude less. This specific ability to achieve remarkably high ion concentrations permits us to engineer bespoke surfaces which have been unachievable in the past for light guiding applications. Such an approach also allows us to engineer the dielectric and spectroscopic properties in the implanted layer. For example, the implanted layer may be doped with rare-earth ions for engineering lasers and amplifiers, but also be vertically integrated with relevant mirrors and photo-active/sensitive materials, e.g. a metal, polymer, semiconductor, ferro-electric ceramic for frequency conversion and manipulation, a biological surface with a protein. A multitude of optically active and passive functions may be achievable via a combination of chemically dissimilar materials on to a dielectric surface (glass, polymer and ceramic).

According to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein

-   (i) the ion implanted layer has a uniform distribution of the     implanted ions; -   (ii) wherein the penetration depth of the implanted ions is at least     200 nm, for example at least 500 nm.

According to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein

-   (i) the ion implanted layer has a uniform distribution of the     implanted ions; and -   (ii) the implanted ion density is at least 10²¹ ions cm⁻³.

According to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein

-   (i) the penetration depth of the implanted ions is at least 200 nm,     for example at least 500 nm; and -   (ii) the implanted ion density is at least 10²¹ ions cm³.

According to a further aspect of the invention there is provided a substrate comprising an ion-implanted layer wherein

-   (i) the ion implanted layer has a uniform distribution of the     implanted ions; -   (ii) wherein the penetration depth of the implanted ions is at least     200 nm; and -   (iii) the implanted ion density is at least 10²¹ ions cm⁻³.

The penetration depth of the ion layer depends on the substrate used but in general would be at least 250 nm, at least 300 nm, at least 400 nm, at least 500 nm, at least 750 nm, at least 1 μm, at least 1.5 μm, at least 2.0 μm, at least 2.5 μm and at least 3 μm. In one embodiment the layer has a depth from about 0 nm to about 3 μm, for example in the range about 0 nm to about 2.5 μm, about 0 nm to about 2 μm, about 0 nm to about 1.5 μm or about 0 nm to about 1 μm. In a further embodiment the layer has a depth from about 500 nm to about 3 μm, for example in the range about 500 nm to about 2.5 μm, about 500 nm to about 2 μm, about 500 nm to about 1.5 μm or about 500 nm to about 1 μm.

In general the implanted ion density is at least 5×10²¹ ions cm⁻³

The ion implanted layer may comprise one or more ions.

The implanted layer may be on an outside face of the substrate or may comprise a layer within the substrate. The implanted layer may comprise one layer or may comprise two or more layers with a distinct combination of ion compositions in each layer. For the avoidance of doubt where there are more than two layers, two or more non-adjacent layers may have the same ion composition, for example, to form a sandwich. Multiple dissimilar materials as targets can be envisaged for tailoring the ion layer composition.

The implanted layer can encompass substantially the whole area of the substrate or can comprise one or more zones. The zones may comprise distinct areas on or within the substrate or one or more of the zones may overlap. The zones may comprise the same ion or ions or one or more of the zones may comprise a different ion or ions.

The ion may be a cation.

The ion may be selected from any cation which is ionisable, for example one or more of the following groups:

-   -   (i) one or more pre-lanthanide and/or lanthanide ions;     -   (ii) erbium, ytterbium, neodymium, praseodymium, holmium,         cerium, yttrium, samarium, europium, gadolinium, terbium,         dysprosium or lutetium ions;     -   (iii) Nd(3+), Yb(3+), Er(3+), Tm(3+), Pr(3+), Ho(3+), Sm(3+),         Eu(3+), Tb(3+) and Ce(3+), La (3+) ions;     -   (iv) tellurium, germanium, zinc, sodium and erbium ions;     -   (v) metallic ions: for example, Bi, W, Nb, Ta, Ti, Mo, Cr, Mn,         Ga, In, Sn, Pb     -   (vi) one or more one or more actinide ions;     -   (vii) actinium, americium, berkelium, californium, curium,         einsteinium, fermium, lawrencium, mendelevium, neptunium,         nobelium, plutonium, protactinium, thorium and uranium,     -   (viii) one or more transition metals;     -   (ix) one or more cations selected from the groups (i) to (ix)         above.     -   (x) One or more anions which may be of F⁻, Cl⁻, Br⁻, I and         chalcogen ions (S and Se).

The silicon-based substrate may comprise silicon, glass, silicon oxide or silicon hydride, siloxane polymer.

In a further embodiment the silicon-based substrate is glass. Example of glass include: silica, silicate, phosphate, tellurite, tellurite derivatives, germanate, bismuthate and solgel route glasses.

The polymeric substrate may comprise Poly(methyl methacrylate) (PMMA), polyvinyl alcohol (PVA), polyether ether ketone (PEEK), polyethylene terephthalate (PET), polyimide(PI), polypropylene (PP), and polytetrafluoroethylene (PTFE).

Novel ion-implanted substrates of the invention have application in a number of technologies, for example, in communication, computer or display technology and in laser assemblies. The novel ion-implanted substrate of the invention may be used in integrated optics (eg as a signal source), chemical sensing, environmental sensing, biosensing, micro-nano spectroscopy, optical communication, micro fluidic devices, optofluidic devices, terahertz amplifiers, lab-on-chip or optical tomography.

The novel ion-implanted substrate of the invention may be used as a waveguide.

In one embodiment of the invention there is provided a waveguide comprising a ion-implanted substrate of the invention.

The wavelength range of fluorescence in a chip comprising a substrate of the invention is governed by the ion which are implanted into the substrate. The skilled man would be familiar with the spectral properties of suitable doping ions and therefore chose the appropriate ions for preparing a ion-implanted substrate with suitable spectral properties.

Example of the spectral properties include”

(i) 1100 nm Yb; (ii) 1530-1580 range Er³⁺; (iii) 1450-1525 and 1725-1900 Thulium; (iv) 1900-2000 nm Ho³⁺; (v) 1750 nm-2200 nm codoping with Tm³⁺ and Ho³⁺ions; (vi) 1100 nm-2200 nm codoping with Yb³⁺, Er³⁺, Tm³⁺ and Ho³⁺;

A biosensor system comprising

-   (i) a laser; -   (ii) a waveguide comprising a substrate of the invention; and -   (iii) a detector.

A biosensor system comprising

-   (i) a laser; -   (ii) a photonic chip comprising an ion-implanted substrate of the     invention; and -   (iii) a photonic chip integrator.

When used in waveguides of the invention the substrate may have a thickness in the range of about 0.1 mm to about 10 mm, such as in the range of 0.5 mm to about 3 mm.

In one embodiment the detector is a fast photodiode. In one embodiment the photodiode is a microsecond photodiode, in a further embodiment a nanosecond photodiode.

One use of ion-implanted substrates of the invention is in the non-invasive detection of metabolites, such as glucose, in animals, such as in a novel method which measures photoluminescence lifetime.

In this method the photoluminescence spectral band of the dopant(s) overlaps with the characteristic absorption bands in the 1530-2200 nm) of the glucose molecules in the NIR wavelengths. The measured photoluminescence lifetime of the rare earth ions gets modified in the glass thin film contained in a photonic sensor due to the amplification by random scattering and localization of photoluminescent photons. When a medium containing glucose interacts with the film, the lifetime modifies as a function of glucose concentration due to its specific absorption as well as molecular scattering properties. Thus by an accurate measurement of the photoluminescence lifetime the concentration of glucose in the medium can be deduced. Since the absorption and scattering properties of the photons at different wavelengths within the emission band varies glucose concentration, the change in the photoluminescence lifetime at different wavelengths can be used as an additional feature to enhance the signal due to glucose from other interferences. This new measurement concept is named as Spectrally Resolved Photoluminescence Lifetime (SRPL) technique and is the novel principle of detection that is applied in the photonic sensor. This new approach avoids the disadvantages of direct spectroscopic methods such as low sensitivity due to strong signal absorption and augments the capabilities of Multisensor in continuous glucose monitoring.

To describe the principle of the sensor based on lifetime measurement, the photonic chip and skin can be treated as a combined scattering medium to because of their characteristically similar scattering properties. While the skin acts as scattering as well as absorption medium, gain or amplification of the optical signal is mainly provided by the doped thin film which is under laser (980 nm) excitation. The glucose molecules can interact with the NIR photons generated from the chip in two ways, wavelength dependent scattering and absorption. Since these molecules are being dispersed in an inhomogeneous medium like skin, the scattering by glucose as well as the medium plays an important role in the photon dynamics. While the scattering by glucose molecules is incoherent (Rayleigh scattering) and that by skin inhomogeneities can result in coherent photons. Any measured increase in the lifetime of the signal photons can be attributed to it being traveled longer paths in the medium as a result of increased probability of coherent scattering. In a simple description this means that the coherent scattering leads to longer measured lifetime while incoherent scattering by molecules reduces lifetime of the measured signal as discussed below. Interestingly, the sensor principle is inherent in the Rayleigh's scattering law[^(i)] which states the scattering cross section (a) by molecules is inversely proportional the fourth power wavelength (λ) and square of the number of molecules (N) i.e.,

$\begin{matrix} {\sigma \propto \frac{1}{\lambda^{4}N^{2}}} & (1) \end{matrix}$

Thus at a particular wavelength, assuming variations in skin's physiological parameters are negligible, the larger the number of glucose molecules lower will be the incoherent scattering cross section. Hence photons available for coherent scattering increases leading to an increase in PL lifetime evident from measured wavelength. FIG. 7 demonstrate this trend slope up to a concentration of ˜10 mMol/L for three different wavelengths when a ˜1 μm thick film was used for measurement in blood samples under controlled conditions. At concentrations higher than this however the trend reverses and it can attributed increasing competition between scattering and absorption. Glucose has a flat absorption response (˜7×10⁻⁵ mM⁻¹ mm⁻¹) within 1534-1580 nm region, therefore at longer wavelengths the reduction in the scattering cross-section will be at a higher magnitude due to the 4^(th) power dependence of λ. At this context, the light photon-liquid medium interaction operates mainly on the absorption phenomenon where photons are lost leading to PL lifetime reduction. This phenomenon generally manifests in large increase (100's of microseconds) measured lifetime in the hypoglycaemic region and even upto concentration levels of 10 mMol/l and therefore results in excellent sensitivity.

In one embodiment of the invention there is provided a method for the non-invasive measurement of a metabolite in an animal which comprises:

-   (i) applying a detector on or near said animal, for example apply     said detector to the skin of the animal, said detector comprising a     wave guide; -   (ii) irradiating said waveguide with a light source, for example, a     laser, such that a portion of the light escapes into the animal; -   (iii) measuring the photoluminescence lifetime of the escaped light     wherein the recovery lifetime is correlated with the level of the     metabolite.

In a further embodiment the change in the photoluminescence lifetime at different wavelengths can be used as an additional feature to enhance the signal due to a metabolite of interest from other interferences.

A laser suitable for use in biosensor systems and detection methods of the invention include a laser in the visible and near infra red spectrum. For example, a laser with wavelengths in the near infra red spectrum such as those with wavelengths from about 780 to about 1300 nm, such as in the range about 980 nm to about 1100 nm, for example about 980 nm. In one embodiment the laser is a tunable continuous wave (cw) random laser within the range of about 1510 to about 1620 nm.

In one embodiment the power of said laser is in the range 1 mW to 500 mW, such as about 50 mW.

The light source, for example, a laser can have an angle of incidence at the waveguide of 1° to 90°, for example, about 45°.

When measuring the recovery lifetime in detection methods of the invention the light source, for example, a laser is turned on an off, for example with a frequency of about 1 Hz to about 10 Hz.

The detection technology of the invention provides wide detection bandwidth, for example, ranging from 800 to 4000 nm.

Waveguides suitable for method of non-invasive measurement of the invention may comprise ion-implanted substrates of the invention, but would also include any waveguide which facilitates transmission of light and retrieval of a portion of said light from a biological material sufficient to measure the recovery lifetime.

Metabolites which can be detected by the method of the invention include: small molecule metabolites, peptides, lipids, peptides, polypeptides and proteins.

Small molecules metabolites include: glucose and lactate.

The photoluminescence spectral band of the dopant(s) overlaps with the characteristic absorption bands of the metabolite or metabolites of interest. For example, in the case of detection of glucose a spectral band of about 1530 to about 2200 nm) of the glucose molecules would be acceptable.

In a further embodiment of the invention there is provided a process for making a ion-implanted substrate of the invention comprising:

ablating a target layer with incident radiation from an ultrafast laser in the presence of a substrate whereby to implant a quantity of the target layer in the substrate.

The process further comprises one or more masks to facilitate implantation of ions in specific zones of the substrate.

The target layer can be any material which when exposed to incident radiation produces a plasma comprising ions capable of implanting into the substrate. Examples of target layers include tellurium glass.

In one embodiment the target layer is mounted on a rotational platform.

The substrate is spaced apart from the target layer, for example at a distance in the range about 50 mm to 150 mm, for example a distance of about 70 mm

The substrate may be heated to facilitate the implantation of ions into the substrate. The optimum temperature to facilitate the implantation of ions will depend on the target material used. In general the optimum temperature is between 0.5 and 0.75 times the glass transition temperature. In embodiment, when using silica glass a temperature of about 973K can be used.

For example, the silica glass transition temperature is 1100-1200° C. Therefore the optimum range is about 550° C.-900° C. (823K-1173K).

The ultrafast laser may be a attosecond, femtosecond or picosecond laser. In one embodiment the ultrafast laser is a femtosecond laser.

The ultrafast laser may be, for example, a Ti-sapphire laser, a diode pumped laser such as a Yb-doped or Cr-doped crystal laser or a fibre laser.

The laser may be an excimer laser or an exciplex laser.

The ultrafast laser may be a pulsed laser.

In the process of the invention, the ultrafast laser may emit pulses of 15 ps or less for example pulses in the range 1 fs to 15 ps. In one embodiment in the process of the invention the ultrafast laser emits pulses of 150 fs or less, for example in the range about 50 to about 150 fs, for example about 100 fs.

The pulses may be emitted with a repetition rate in the range about 400 Hz to about 1 kHz In one embodiment 400 Hz to 800 kHz, for example about 500 kHz.

The ultrafast laser may be mode-locked.

The average power of the ultrafast laser may be 80 W or less.

The pulse energy is typically in the range 40 to about 80 microjoules, for example about 50 to about 70 microjoules, such as about 65 microjoules.

Pulse energy may be selectively adjusted using an attenuator.

In one embodiment wavelength is typically about 800 nm, although a wide range of wavelengths would be suitable.

The incident radiation may be incident on the target glass at an angle in the range about 40° to about 80°, for example about 60°.

The process is typically carried out in a vacuum chamber. The process may be carried out at reduced pressure, for example at a partial pressure of about 60 mTorr.

The process may be conducted for example in the presence of a gas, such as oxygen or in an inert gas.

The duration of the process may be about 30 minutes or more, for example about 30 minutes to about 10 hours, such as about 2 hours to about 8 hours. In one embodiment the duration is about 6 hours.

In a further embodiment of the invention there is provided a process for making a ion-implanted substrate of the invention comprising:

-   (i) providing a target layer; -   (ii) providing s substrate in proximity to said target layer; and -   (iii) directing incident radiation from an ultrafast laser at the     target layer to produce an ion-comprising plasma whereby ions from     said plasma are implanted into the substrate.

The term ‘about’ when used in this specification refers to a tolerance of ±10%, of the stated value, i.e. about 50% encompasses any value in the range 45% to 55%, In further embodiments ‘about’ refers to a tolerance of ±5%, ±2%, ±1%, ±0.5%, ±0.2% or 0.1% of the stated value.

The term ‘animal’ includes mammals, such as humans.

The term ‘dopants’ refers to ions implanted into a substrate. Dopants include ions implanted into substrates of the invention.

The term ‘glass’ refers to a solid that possesses a non-crystalline (i.e., amorphous) structure and that exhibits a glass transition when heated towards the liquid state and which transmits light in the infrared, visible or ultraviolet spectrum, i.e. a wavelength of about 10 nM to 300 μm. In one embodiment ‘glass’ refers to a glass which transmits light in the visible spectrum i.e. a wavelength of about 380 nm to about 740 nm. In a further embodiment ‘glass’ refers to a glass which transmits light in the infrared spectrum i.e. a wavelength of about 740 nm to about 300 μm. In a further embodiment ‘glass’ refers to a glass which transmits light in the ultraviolet spectrum i.e. a wavelength of about 10 nm to about 380 nm. In a yet further embodiment ‘glass’ refers to a glass which transmits light in the wavelength range about 400 nm to about 2000 nm.

The term ‘implantation’ refers to ion entering the matrix of the substrate rather than forming a film on the surface of the substrate.

The term ‘optical polymer’ refers to any polymer which transmits light in the infrared, visible or ultraviolet spectrum, i.e. a wavelength of about 10 nM to 300 μm. In one embodiment ‘optical polymer’ refers to a polymer which transmits light in the visible spectrum i.e. a wavelength of about 380 nm to about 740 nm. In a further embodiment ‘optical polymer’ refers to a polymer which transmits light in the infrared spectrum i.e. a wavelength of about 740 nm to about 300 μm. In one embodiment ‘optical polymer’ refers to a polymer which transmits light in the ultraviolet spectrum i.e. a wavelength of about 10 nm to about 380 nm. In a yet further embodiment ‘optical polymer’ refers to optical polymers which transmits light in the wavelength range about 400 nM to about 2000 nm.

The term ‘substrate’ refers to a silicon-based substrate or a polymeric substrate, for example, a material selected from glass or an optical polymer.

The term ‘waveguide’ refers to any element which facilitates transmission of light into a material of interest and facilitates measurement of light which is retrieved from the material of interest.

The invention will now be illustrated with the following non-limiting examples with reference to the following figures.

FIG. 1—shows schematically the ablation, plasma production and the multi-ion diffusion process.

FIG. 2—shows the SEM and TEM images of the substrate cross sections with a highly defined and uniformly diffused region in silica at two different target ablation energies of 47 μJ (Sample A) and 63 μJ (Sample B) respectively.

FIG. 3—shows a 400×1600 nm HAADF slices of individual elements of Sample B.

FIG. 4 represent the Raman spectrum of ion diffused glass compared with bare silica and tellurite bulk glass.

FIG. 5 represents a schematic diagram of a biosensor, such as a glucose sensor of the invention.

FIG. 6 shows Molar absorptivity spectra of glucose (solid), alanine (dashdot-dot), ascorbate (medium dash), lactate (short dash), urea (dotted), and triacetin (dash-dot) at 37.0±0.1° C. over the first overtone.

FIG. 7 shows the variation in photoluminescence lifetime measured at three different wavelengths for human blood sample with varying concentrations of glucose

ABBREVIATIONS USED

-   HAADF high angle angular dark field elemental mapping -   NIR near infra red -   SEM Scanning electron microscopy -   TEM Transmission electron microscopy

EXAMPLE 1 Implantation into Silica Glass

Multi-ion diffusion into silica glass was produced via femtosecond laser ablation of an erbium doped tellurite glass target containing zinc and sodium. A Ti-sapphire femtosecond laser operating at a wavelength of 800 nm with 100 fs pulse width and a maximum repetition rate of 1 kHz (Coherent Inc, Santa Clara, Calif., USA) was used to ablate the glass target generating an expanding plasma plume consisting of multiple metal ions (multi-ion). A tellurite glass target with a molar composition of 79.5TeO₂: 10ZnO:10Na₂O:0.5Er₂O₃ produces multiple ions Te4+, Zn2+, Na+ and Er3+, which diffuse into the silica glass substrate under certain process conditions. The ablation, plasma production and the multi-ion diffusion process are schematically shown in FIG. 1.

Experiments were carried by varying the laser energy, repetition rate, target to substrate distance and finally the deposition target temperature. The deposition target was not translated for the simplicity of the experiment and for a better understanding of parameter variation along the sample surface. There was an variation in diffusion depth and refractive index profile along the surface when radially moving outwards from the centre, therefore all the characteristic properties of the modification provided were measured from the centre of the sample unless otherwise stated.

Optimum results were obtained for laser energies between 40 μJ-75 μJ when operated at 500 Hz and 1 kHz. The ablation target to substrate distance was set at 70 mm and the substrate temperature was set at 973K. FIG. 2 represents the SEM and TEM images of the substrate cross sections with a highly defined and uniformly diffused region in silica at two different target ablation energies of 47 μJ (Sample A) and 63 μJ (Sample B) respectively. Diffusion depths of the ions increased from 350 nm to 850 nm with laser energy while the deposition time was 6 hours and repetition rate was 500 Hz for both cases. A well-defined boundary of the diffused and pristine region is clearly visible in the FIG. 2 and the modified region does not show any major clustering of ions or particle inhomogeneities.

Further analysis of the diffusion characteristics of each ions in silica was carried out using high angle angular dark field (HAADF) elemental mapping of sample B. FIG. 3 depicts a 400×1600 nm HAADF slices of individual elements. A line intensity profile shows the relative concentration profile of each diffused elements with a well-defined and sharp boundary within the silica. The oxygen concentration in silica remained unchanged across the boundary while silicon showed a complementary concentration profile with respect to the diffused elements. This indicates the formation of a complex alloy glass of silica with implanted ions increasing refractive index from 1.457 of that of silica to 1.626. The atomic concentration of silicon in the diffused region is determined to be 57% while Te, Zn, Na and Er constitute the rest in Sample A. This confirms a single step multi-ion diffusion process in the silica glass substrate. The diffusion is highly uniform and homogenous along the transverse and horizontal sections of the silica substrate.

Structural Properties of Diffused Region:

Silica and tellurite are completely immiscible and will not form a stable glass under conventional batch melting and quenching process. However in the results presented above it is demonstrated that diffusion of metal ions including Te4+ ions in to the silica glass network is possible. The properties of the implanted silica glass were measured. No signals of any kind of crystallization were observed in electron diffraction and XRD characterization proving a complete amorphous phase of silica-tellurite glass. Raman spectroscopy was used to analyse the glass network in the diffused region. FIG. 4 represent the Raman spectrum of ion diffused glass compared with bare silica and tellurite bulk glass. The TZN bulk glass shows typical raman spectrum with peaks at 817 cm⁻¹ (TeO₃₊₁ and TeO₃ stretching), 653 cm⁻¹ (TeO₄ stretching) and 520 cm⁻¹ (Te—O—Te bending). On analysing the Raman spectrum of the implanted region on the substrate, a broad peak corresponding to TeO₄ stretching vibrations are found within the range 600-668 cm⁻¹. The reduction in intensity is due to the distortion and destruction of Tea₄ groups. This observation is supplemented by the fact that the 794 cm⁻¹ vibration is increased with the formation of more TeO₃₊₁ and TeO₃ groups. The 490 cm⁻¹ peak which is very weak in silica substrate shows a strong and sharp response in the implanted glass. This indicates the post-implant state of Si—O—Si bonds, 490 cm⁻¹ peak in υ-SiO₂ was experimentally demonstrated to increase its intensity with elastic tensile stress and later explained that this is arising from the four-membered ring structures in υ-SiO₂. Hence the vibration becomes stronger when their concentration increases upon increase in density of υ-SiO₂. The broadness in the peak found at 600-668 cm⁻¹ may take up contribution from the reported 604 cm⁻¹ peak in υ-SiO₂ due to stretched Si—O bonds during irradiation process. 

1. A substrate comprising an ion-implanted layer wherein the penetration depth of the implanted ions is at least 50 nm, or at least 200 nm.
 2. A substrate according to claim 1 where the penetration depth of the implanted ions is at least 500 nm.
 3. A substrate according to claim 1 wherein the ion implanted layer has a substantially uniform distribution of the implanted ions.
 4. A substrate according to claim 1 wherein the ion implanted layer has an implanted ion density of at least 10²¹ ions cm⁻³, or at least 10²³ ions cm⁻³.
 5. A substrate according to claim 1 wherein the substrate is a glass selected from silica, silicate, phosphate, tellurite, tellurite derivatives, germanate, bismuthate and solgel route glasses.
 6. A substrate according to claim 1 wherein the substrate is an optical polymer.
 7. A substrate according to claim 6 wherein the optical polymer is selected from Poly(methyl methacrylate), polyvinyl alcohol, polyether ether ketone, polyethylene terephthalate, polyimide, polypropylene, and polytetrafluoroethylene.
 8. A substrate according to claim 1 wherein the ion-implanted layer is either: (i) on an outside face of the substrate; or (ii) within the substrate.
 9. A substrate according to claim 1 wherein the ion-implanted layer either: (i) encompasses substantially the whole area of the substrate; or (ii) comprises one or more zones.
 10. A substrate according to claim 9 wherein one or more of the zones overlap.
 11. A substrate according to claim 9 wherein the zones comprise the same or different ions.
 12. A substrate according to claim 1 wherein the ion is a cation.
 13. A substrate according to claim 12 wherein the cation is selected from the group Nd(3+), Yb(3+), Er(3+), Tm(3+), Pr(3+), Ho(3+), Sm(3+), Eu(3+), Tb(3+), Ce(3+) and La (3+).
 14. A waveguide comprising a substrate according to claim
 1. 15. A biosensor comprising a substrate according to claim 1 as an optical substrate or as a waveguide comprising said substrate.
 16. A method for the non-invasive measurement of a metabolite in an animal which comprises: (i) applying a sensor on or near said animal, said sensor comprising an optical substrate or waveguide; (ii) irradiating said substrate or waveguide with a light source such that a portion of the light escapes into the animal; (iii) measuring the photoluminescence lifetime of the escaped light; wherein the recovery lifetime is correlated with the level of the metabolite.
 17. A process for fabricating a substrate according to claim 1 comprising: ablating a target layer with incident radiation from a laser in the presence of a substrate whereby a quantity of the target layer is implanted into the substrate.
 18. A process according to claim 17 wherein the target layer is tellurium glass.
 19. A process according to claim 17 wherein the laser is a Femtosecond laser.
 20. A process according to claim 17 wherein the substrate is heated.
 21. A substrate comprising an ion-implanted layer wherein the ion implanted layer has a substantially uniform distribution of the implanted ions.
 22. A substrate comprising an ion-implanted layer wherein the ion implanted layer has an implanted ion density of at least 10²¹ ions cm⁻³, or at least 10²³ ions cm⁻³. 